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This module provides mechanisms to use signal handlers in Python. Some
general rules for working with signals and their handlers:

A handler for a particular signal, once set, remains installed until
it is explicitly reset (Python emulates the BSD style interface
regardless of the underlying implementation), with the exception of
the handler for SIGCHLD, which follows the underlying
implementation.

There is no way to “block” signals temporarily from critical
sections (since this is not supported by all Unix flavors).

Although Python signal handlers are called asynchronously as far as
the Python user is concerned, they can only occur between the
“atomic” instructions of the Python interpreter. This means that
signals arriving during long calculations implemented purely in C
(such as regular expression matches on large bodies of text) may be
delayed for an arbitrary amount of time.

When a signal arrives during an I/O operation, it is possible that
the I/O operation raises an exception after the signal handler
returns. This is dependent on the underlying Unix system’s semantics
regarding interrupted system calls.

Because the C signal handler always returns, it makes little sense
to catch synchronous errors like SIGFPE or SIGSEGV.

Python installs a small number of signal handlers by default:
SIGPIPE is ignored (so write errors on pipes and sockets can be
reported as ordinary Python exceptions) and SIGINT is translated
into a KeyboardInterrupt exception. All of these can be
overridden.

Some care must be taken if both signals and threads are used in the
same program. The fundamental thing to remember in using signals
and threads simultaneously is: always perform signal()
operations in the main thread of execution. Any thread can perform
an alarm(), getsignal(), pause(), setitimer() or
getitimer(); only the main thread can set a new signal handler,
and the main thread will be the only one to receive signals (this is
enforced by the Python signal module, even if the underlying
thread implementation supports sending signals to individual
threads). This means that signals can’t be used as a means of
inter-thread communication. Use locks instead.

The variables defined in the signal module are:

signal.SIG_DFL

This is one of two standard signal handling options; it will simply
perform the default function for the signal. For example, on most
systems the default action for SIGQUIT is to dump core and
exit, while the default action for SIGCHLD is to simply ignore
it.

signal.SIG_IGN

This is another standard signal handler, which will simply ignore
the given signal.

SIG*

All the signal numbers are defined symbolically. For example, the
hangup signal is defined as signal.SIGHUP; the variable names
are identical to the names used in C programs, as found in
<signal.h>. The Unix man page for ‘signal()‘ lists the
existing signals (on some systems this is signal(2), on others
the list is in signal(7)). Note that not all systems define the
same set of signal names; only those names defined by the system
are defined by this module.

signal.NSIG

One more than the number of the highest signal number.

signal.ITIMER_REAL

Decrements interval timer in real time, and delivers SIGALRM
upon expiration.

signal.ITIMER_VIRTUAL

Decrements interval timer only when the process is executing, and
delivers SIGVTALRM upon expiration.

signal.ITIMER_PROF

Decrements interval timer both when the process executes and when
the system is executing on behalf of the process. Coupled with
ITIMER_VIRTUAL, this timer is usually used to profile the time
spent by the application in user and kernel space. SIGPROF is
delivered upon expiration.

The signal module defines one exception:

exception exception signal.ItimerError

Raised to signal an error from the underlying setitimer() or
getitimer() implementation. Expect this error if an invalid
interval timer or a negative time is passed to setitimer().
This error is a subtype of IOError.

The signal module defines the following functions:

signal.alarm(time)

If time is non-zero, this function requests that a SIGALRM
signal be sent to the process in time seconds. Any previously
scheduled alarm is canceled (only one alarm can be scheduled at any
time). The returned value is then the number of seconds before any
previously set alarm was to have been delivered. If time is zero,
no alarm is scheduled, and any scheduled alarm is canceled. If the
return value is zero, no alarm is currently scheduled. (See the
Unix man page alarm(2).) Availability: Unix.

signal.getsignal(signalnum)

Return the current signal handler for the signal signalnum. The
returned value may be a callable Python object, or one of the
special values signal.SIG_IGN, signal.SIG_DFL or None.
Here, signal.SIG_IGN means that the signal was previously
ignored, signal.SIG_DFL means that the default way of handling
the signal was previously in use, and None means that the
previous signal handler was not installed from Python.

signal.pause()

Cause the process to sleep until a signal is received; the
appropriate handler will then be called. Returns nothing. Not on
Windows. (See the Unix man page signal(2).)

signal.setitimer(which, seconds[, interval])

Sets given interval timer (one of signal.ITIMER_REAL,
signal.ITIMER_VIRTUAL or signal.ITIMER_PROF) specified by
which to fire after seconds (float is accepted, different from
alarm()) and after that every interval seconds. The interval
timer specified by which can be cleared by setting seconds to
zero.

When an interval timer fires, a signal is sent to the process. The
signal sent is dependent on the timer being used;
signal.ITIMER_REAL will deliver SIGALRM,
signal.ITIMER_VIRTUAL sends SIGVTALRM, and
signal.ITIMER_PROF will deliver SIGPROF.

The old values are returned as a tuple: (delay, interval).

Attempting to pass an invalid interval timer will cause a
ItimerError.

New in version 2.6.

signal.getitimer(which)

Returns current value of a given interval timer specified by
which.

New in version 2.6.

signal.set_wakeup_fd(fd)

Set the wakeup fd to fd. When a signal is received, a '\0'
byte is written to the fd. This can be used by a library to wakeup
a poll or select call, allowing the signal to be fully processed.

The old wakeup fd is returned. fd must be non-blocking. It is
up to the library to remove any bytes before calling poll or select
again.

When threads are enabled, this function can only be called from the
main thread; attempting to call it from other threads will cause a
ValueError exception to be raised.

signal.siginterrupt(signalnum, flag)

Change system call restart behaviour: if flag is False,
system calls will be restarted when interrupted by signal
signalnum, otherwise system calls will be interrupted. Returns
nothing. Availability: Unix (see the man page siginterrupt(3) for
further information).

Note that installing a signal handler with signal() will reset
the restart behaviour to interruptible by implicitly calling
siginterrupt() with a true flag value for the given signal.

New in version 2.6.

signal.signal(signalnum, handler)

Set the handler for signal signalnum to the function handler.
handler can be a callable Python object taking two arguments (see
below), or one of the special values signal.SIG_IGN or
signal.SIG_DFL. The previous signal handler will be returned
(see the description of getsignal() above). (See the Unix man
page signal(2).)

When threads are enabled, this function can only be called from the
main thread; attempting to call it from other threads will cause a
ValueError exception to be raised.

The handler is called with two arguments: the signal number and
the current stack frame (None or a frame object; for a
description of frame objects, see the reference manual section on
the standard type hierarchy or see the attribute descriptions in
the inspect module).

Here is a minimal example program. It uses the alarm() function to
limit the time spent waiting to open a file; this is useful if the
file is for a serial device that may not be turned on, which would
normally cause the os.open() to hang indefinitely. The solution
is to set a 5-second alarm before opening the file; if the operation
takes too long, the alarm signal will be sent, and the handler raises
an exception.